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Pro Straightforward interface
The interface is very simple, fast and easy to use. You can start using it as soon as the app has been downloaded - it does not require setting anything up or registering. You can communicate with anyone in your contacts list that also uses WhatsApp. Includes useful features like seeing when a contact is currently available or typing.
Pro Effortlessly builds your contacts by using your phone number as identification
WhatsApp will automatically find all of your contacts currently using Whatsapp and add them to your contacts. This makes it really easy to setup and given that it's the most popular messaging app it's likely that a lot of your contacts are already using it.
Pro Widely used
WhatsApp reported 800 million users in April, 2015. Your friends are most likely already using WhatsApp, so it's easy to get setup. Also it's the overall highest rated messaging app on the play store.
Pro The client can be mirrored on the desktop
Pro One-on-one texts can be encrypted
WhatsApp has implemented end to end encryption for messages sent on it's service. Even WhatsApp has no way to read a users messages.
Con Hard to migrate history cross platform
Migrating your chat history from Android to iOS or the other way around is not so simple. In some cases it requires a 3rd party app and in some the only way is to migrate specific chats one by one.
Con Account is limited to one device
You cannot use the same whatsapp account on multiple devices (e.g. a tablet, a PC and a cellphone). The account is tied to the device with the specific cellphone number. That also means that there's no sync since it doesn't keep conversations when switching operating systems or devices.
Con Doesn't support file sharing
It doesn't let you share files, only pics, vids or music.
Not open source.
Con Encrypted? Pfff, barely any key checking
Whatsapp say's everything is encrypted and, yes it is.
But if you turn your phone off, and someone else pretends to be your phone, he or she will receive your messages 2 days later without any security breach warnings or what so ever.
The message's are delivered without any problems, so nice they have encryption, but pretty useless if the keys are not really checked.